In-line electron gun

ABSTRACT

The three co-planar beams of an in-line gun are converged near the screen of a cathode ray tube by means of two plate-like grids transverse to the beam paths and having corresponding apertures for the three beams. The three beam apertures of the first grid are aligned with the three beam paths. The two outer beam apertures of the second grid are offset outwardly relative to the beam paths to produce the desired convergence. The three sets of apertures also provide separate focusing fields for the three beams. The second plate-like grid is formed with a barrel shape, concave toward the first grid, to minimize elliptical distortion of beam spots on the screen due to crowding of the adjacent focusing fields. Each of the two outer beams is partially shielded from the magnetic flux of the deflecting yoke by means of a magnetic ring surrounding the beam path in the deflection zone, to equalize the size of the rasters scanned on the screen by the middle and outer beams. Other magnetic pieces are positioned on opposite sides of the path of the middle beam, to enhance one deflection field while reducing the transverse deflection field for that beam.

313*414g XR NOV. 13, 1973 IN-LINE ELECTRON GUN [75] Inventor: Richard Henry Hughes, Lancaster,

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Jan. 14, 1972 [2]] Appl. No.: 217,758

[52] US. Cl. 313/69 C, 313/70 C [51] Int. Cl. 1'10lj 29/50 [58] Field 01 Search 313/70 C, 69 C [56] References Cited UNITED STATES PATENTS 2,412,687 12/1946 Klemperer 313/86 2,849,647 8/1958 Francken 313/70 C X 2,859,378 11/1958 Gundert et a1... 313/70 C X 2,887,598 5/1959 Benway 313/70 C 2,957,106 10/1960 Moodey 313/70 C X 3,164,737 1/1965 Messineo et a1. 313/69 C 3,196,305 7/1965 Barkow 313/69 C 3,534,208 10/1970 Krackhardt et a1. 313/70 C X 3,678,316 7/1972 Barten 313/69 C 3,678,320 7/1972 Barten 313/82 BF Primary ExaminerRoy Lake Assistant Examiner-James B. Mullins Attorney-Glenn H. Bruestle et a l.

i 57] ABSTRACT The three co-planar beams of an in-line gun are converged near the screen of a cathode ray tube by means of two plate-like grids transverse to the beam paths and having corresponding apertures for the three beams. The three beam apertures of the first grid are aligned with the three beam paths. The two outer beam apertures of the second grid are offset outwardly relative to the beam paths to produce the desired convergence. The three sets of apertures also provide separate focusing fields for the three beams. The second plate-like grid is formed with a barrel shape, concave toward the first grid, to minimize elliptical distortion of beam spots on the screen due to crowding of the adjacent focusing fields. Each of the two outer beams is partially shielded from the magnetic flux of the deflecting yoke by means of a magnetic ring surrounding the beam path in the deflection zone, to equalize the size of the rasters scanned on the screen by the middle and outer beams. Other magnetic pieces are positioned on opposite sides of the path of the middle beam, to enhance one deflection field while reducing the transverse deflection field for that beam.

11 Claims, 10 Drawing Figures PATENTEDNDV 13 I975 3. 772 554 SHEET 1 CF 4 PATENTED NOV 13 I975 SHEET 3 OF 4 PATENIED NOV 13 I975 SHEET L CF 4 lie. 5

Fin. 0

IN-LINE ELECTRON GUN BACKGROUND OF THE INVENTION The present invention relates to an improved in-line electron gun for a cathode ray tube, particularly a shadow mask type color picture tube. The new gun is primarily intended for use in a color tube having a line type color phosphor screen, with or without light absorbing guard bands between the colorlines, and a mask having elongated apertures or slits. However, the gun could be used in the well known dot-type color tube having a screen of substantially circular color phosphor dots and a mask with substantially circular apertures.

An in-line electron gun is one designed to generate or initiate at least two, and preferably three, electron beams in a common plane, for example, by at least two cathodes, and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. Various ways have been proposed for causing the beams to converge near the screen. For example, the gun may be designed to initially aim the beams, from the cathodes, towards convergence at the screen, as shown in FIG. 4 of the Moodey U.S. Pat. No. 2,957,106, wherein the beam apertures in the gun electrodes are aligned along convergent paths.

In order to avoid wide spacings between the cathodes, which are undesirable in a small neck tube designed for high deflection angles, it is preferable to initiate the beams along substantially parallel (or even divergent) paths and provide some means, either internally or externally of the tube, for converging the beams near the screen. Magnet poles and/or electrostatic deflecting plates for converging in-line beams are disclosed in Francken U.S. Pat. No. 2,849,647, Gundert et al. U.S. Pat. No. 2,859,378 and Benway U.S. Pat. No. 2,887,598.

The Moodey patent referred to above also includes an embodiment, shown in FIG. 2 and described in lines 4 to 23 of column 5, wherein an in-line gun for two coplanar beams comprises two spaced cathodes, a control grid plate and an acceleratinggrid plate each having two apertures aligned respectively with the two cathodes (as in FIG. 2) to initiate two parallel co-planar beam paths, and two spaced-apart beam focusing and accelerating electrodes of cylindrical form. The focusing electrode nearest to the first accelerating grid plate is described as having two beam apertures that are offset toward the axis of the gun from the corresponding apertures of the adjacent accelerating grid plate, to provide an asymmetric electrostatic field in the path of each beam for deflecting the beam from its initial path into a second beam path directed toward the tube axis.

Netherlands Pat. application No. 6,902,025, published Aug. 1 l, 1970 teaches that astigmatic aberration resulting in elliptical distortion of the focused screen spots of the two off-axis beams from an in-line gun, caused by the eccentricity of the in-line beams in a common focusing field between two hollow cylindrical focusing electrodes, can be partially corrected by forming the adjacent edges of the cylindrical electrodes with a sinusoidal contour including four sine waves. A similar problem is solved in a different manner in applicant's in-line gun.

Another problem that exists in a cathode ray tube having an in-line gun is a coma distortion wherein the sizes of the rasters scanned on the screen by a conventional external magnetic deflection yoke are different, because of the eccentricity of the two outer beams with respect to the center of the yoke. Messineo et al. U.S. Pat. No. 3,164,737 teaches that a similar coma distortion caused by using different beam velocities can be corrected by use of a magnetic shield around the path of one or more beams in a delta type gun. Barkow U.S. Pat. No. 3,196,305 teaches the use of magnetic enhancers adjacent to the path of one or more beams in a delta gun, for the same purpose. Krackhardt et al. U.S. Pat. No. 3,534,208 teaches the use of a magnetic shield around the middle one of three in-line beams for coma correction.

SUMMARY OF THE INVENTION In accordance with one aspect of the invention, at least two electron beams are generated along co-planar paths toward the screen of a cathode ray tube, e.g., a shadow mask type color picture tube, and the beams are converged near the screen by asymmetric electric fields established in the paths of two beams by two plate-like grids positioned between the beam generating means and the screen and having corresponding apertures suitably related to the beam paths. The apertures in the first grid (nearest the cathodes) are aligned with the beam paths. Two apertures in the second grid (nearest the screen) are offset outwardly with respect to the beam paths to produce the desired asymmetric fields. In the case of three in-line beams, the two outer apertures are offset, and the middle apertures of the two grids are aligned with each other. The pairs of corresponding apertures also provide separate focusing fields for the beams. In order to minimize elliptical distortion of one or more of the focused beam spots on the screen due to crowding of adjacent beam focusing fields, at least a portion of the second grid may be substantially cylindrically curved in a direction transverse to the common plane of the beams, and concave to the first grid. Each of the two outer beam paths of a three beam gun may be partially shielded from the magnetic flux of the deflection yoke by means of a magnetic ring surrounding each beam in the deflection zone of the tube, to minimize differences in the size of the rasters scanned on the screen by the middle and outer beams. Further correction for coma distortion may be made by positioning magnetic pieces on opposite sides of the middle beam path for enhancing one field and reducing the field transverse thereto.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view, partly in axial section, of a shadow mask color picture tube in which the present invention is incorporated;

FIG. 2 is a front end view of the tube of FIG. 1 showing the rectangular shape;

FIG. 3 is an axial section view of the electron gun shown in dotted lines in FIG. 1, taken along the line 33 of that figure;

FIG. 4 is an axial section view of the electron'gun taken along the line 4-4 of FIG. 3;

FIG. 5 is a rear end view of the electron gun of FIG. 4, taken in the direction of the arrows 5-5 thereof;

FIG. 6 is a transverse view, partly in section, taken along the line 66 of FIG. 4;

FIG. 7 is a front end view of the electron gun of FIGS. 1 and 4;

FIG. 8 is a similar end view with the final element (shield cup) removed; and

FIGS. 9 and are schematic views showing the focusing and converging electric fields associated with two pairs of beam apertures in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a plan view of a 17V-90 rectangular color picture tube, for example, having a glass envelope 1 made up of a rectangular (FIG. 2) faceplate panel or cap 3 and a tubular neck 5 connected by a rectangular funnel 7. The panel 3 comprises a viewing faceplate 9 and a peripheral flange or side wall 11 which is sealed to the funnel 7. A mosaic three-color phosphor screen 13 is carried by the inner surface of the faceplate 9. The screen is preferably a line screen with the phosphor lines extending substantially parallel to the minor axis YY of the tube (normal to the plane of FIG. 1). A multi-apertured color selection electrode or shadow mask 15 is removably mounted, by conventional means, in predetermined spaced relation to the screen 13. An improved in-line electron gun 19, shown schematically by dotted lines in FIG. 1, is centrally mounted within the neck 5 to generate and direct three electron beams along co-planar convergent paths through the mask 15 to the screen 13.

The tube of FIG. 1 is designed to be used with an external magnetic deflection yoke, such as the yoke 21 schematically shown, surrounding the neck 5 and funnel 7, in the neighborhood of their junction, for subjecting the three beams 20 to vertical and horizontal magnetic flux, to scan the beams horizontally and vertically in a rectangular raster over the screen 13. The initial plane of deflection (at zero deflection) is shown by the line P-P in FIG. 1 at about the middle of the yoke 21. Because of fringe fields, the zone of deflection of the tube extends axially, from the yoke 21, into the region of the gun 19. For simplicity, the actual curvature of the deflected beam paths 20 in the deflection zone is notshown in FIG. 1.

The in-line gun 19 of the present invention is designed to generate and direct three equally-spaced coplanar beams along initially-parallel paths to a convergence plane CC, and then along convergent paths through the deflection plane to the screen 13. In order to use the tube with a line-focus yoke 21 specially designed to maintain the three in-line beams substantially converged at the screen without the application of the usual dynamic convergence forces, which cause degrouping misregister of the beam spots with the phosphor elements of the screen, the gun is preferably designed with small spacings between the beam paths at the convergence plane CC to produce a still smaller spacing, usually called the 8" value, between the outer beam paths and the central axis A-A of the tube, in the deflection plane P-P. The convergence angle of the outer beams with the central axis is are tan e/c+d, where c is the axial distance between the convergencc plane CC and the deflection plane PP, d is the distance between the deflection plane and the screen 13, and e is the spacing between the outer beam paths and the central axis A-A in the convergence plane CC. The approximate dimensions in FIG. I are c 2.7 inches, d 9.8 inches, e 0.200 inches (200 mils), and hence, the convergence angle is 55 minutes and s I57 mils.

The details of the improved gun 19 are shown in FIGS. 3 through 8. The gun comprises two glass support rods 23 on which the various electrodes are mounted. These electrodes include three equallyspaced co-planar cathodes 25, one for each beam, a control grid electrode 27, a screen grid electrode 29, a first accelerating and focusing electrode 31, a second accelerating and focusing electrode 33, and a shield cup 35, spaced along the glass rods 23 in the order named.

Each cathode 25 comprises a cathode sleeve 37, closed at the forward end by a cap 39 having an end coating 41 of electron emissive material and a cathode support tube 43. The tubes 43 are supported on the rods 23 by four straps 45 and 47 (FIG. 6). Each cathode 25 is indirectly heated by a heater coil 49 positioned within the sleeve 37 and having legs 51 welded to heater straps 53 and 55 mounted by studs 57 on the rods 23 (FIG. 5). The control and screen grid electrodes 27 and 29 are two closely-spaced (about 9 mils) flat plates having three pairs of small (about 25 mils) aligned apertures 59 centered with the cathode coatings 41 to initiate three equally spaced coplanar beam paths 20 extending toward the screen 13. Preferably, the initial paths 20a and 20b are substantially parallel and about 200 mils apart, with the middle path 20a coincident with the central axis A-A.

Electrode 31 comprises first and second cup-shaped members 61 and 63, respectively, joined together at their open ends. The first cup-shaped member 61 has three medium-sized (about mils) apertures 75 close to grid electrode 29 and aligned respectively with the three beam paths 20, as shown in FIG. 4. The second cup-shaped member 63 has three large (about 160 mils) apertures 65 alsoaligned with the three beam pathsQElectrode 33 is also cup-shaped and comprises a base plate portion 69 positioned close (about 60 mils) to electrode 31 and a side wall or flange 71 extending forward toward the tube screen. The base portion 69 is formed with three apertures 73, which are preferably slightly larger (about 172 mils) than the adjacent apertures 67 of electrode 31. The middle apertures 73a is aligned with the adjacent middle aperture 67a (and middle beam path 20a) to provide a substantially symmetrical beam focusing electric field between apertures 67a and 7311 when electrodes 31 and 33 are energized at different voltages. The two outer apertures 73b are slightly offset outwardly with respect to the corresponding outer apertures 67b, to provide an asymmetrical electric field between each pair of outer apertures when electrodes 31 and 33 are energized, to individually focus each outer beam 20b near the screen, and also to deflect each beam, toward the middle beam, to a common point of convergence with the middle beam near the screen. In the example shown, the offset of each beam aperture 73b may be about 6 mils The approximate configuration of the electric fields associated with the middle and outer apertures are shown in FIGS. 9 and 10, respectively, which show the equipotential lines 74 rather than the lines of force. Assuming an accelerating field, as shown by the signs, the left half 75 (on the left side of the central midplane) of each field is converging and the right half 77 is diverging. Since the electrons are being accelerated, they spend more time in the converging field than in the diverging field, and hence, the beam experiences a net converging or focusing force in each of FIGS. 9 and 10. Since the middle beam 200 passes centrally through a symmetrical field in FIG. 9, it continues in the same direction without deflection. In FIG. 10, the outer beam 20b traverses the left half 75 of the field centrally, but enters the right half 77 off axis. Since this is the diverging part of the field, and the electrons are subjected to field forces perpendicular to the equipotential lines or surfaces 74, the beam 20b is deflected toward the central axis (downward in FIG. as it traverses the right half 77, in addition to being focused. The angle of deflection, or convergence, of the beam 29b can be determined by the choice of the offset of the apertures 73b and the voltage applied to the two electrodes 31 and 33. For the example given, with an offset of 6 mils, electrode 33 would be connected to the ultor or screen voltage, about 25 I(.V., and electrode 31 would be operated at about 17 to 20 percent of the ultor voltage, adjusted for best focus. The object distance of each focus lens, that is, the distance between the first cross-over of the beams near the screen grid 29 and the lens, is about 0.500 inch; and the image distance from the lens to the screen is about l2.5 inches.

The above-described outward offset of the beam apertures to produce beam convergence is contrary to the teaching of FIG. 3 of the Moodey patent described above, and hence, is not suggested by the Moodey patent.

The focusing apertures 67 and 73 are made as large as possible, to minimize spherical aberration, and as close together as possible, to obtain a desirable small spacing between beam paths. As a result, the fringe portions of adjacent fields interact to produce some astigmatic distortion of the focusing fields, which produces some ellipticity of the normally-circular focused beam spots on the screen. In a three-beam in-line gun, this distortion is greater for the middle beam than for the two outer beams, because both sides of the middle beam field are affected. In order to compensate for this effect, and minimize the elliptical distortion of the beam spots, the wall 69, or at least the surface thereof facing the electrode 31, is curved substantially cylindrically, concave to electrode 31, in the direction normal plane transverse to the lane of the three beams, as shown at 79 in FIG. 3. Preferably, this curvature is greater for the middle beam path than for the outer beam paths, hence, the wall 69 may be made barrelshaped. In the example given, the barrel shape may have a stave radius of 8 inches (FIG. 4) and a hoop radius of 2.28 inches (FIG. 3), with the curvature 79 terminating at the outer edges of the outer apertures 73b.

The shield cup 35 comprises a base portion 81, attached to the open end of the flange 71 of electrode 33, and a tubular wall 83 surrounding the three beam paths 20. The base portion 81 is formed with a large middle beam aperture 85 (about 172 mils) and two smaller outer beam apertures 87 (about 100 mils) aligned, respectively, with the three initial beam paths 20a and 20b.

In order to compensate for the coma distortion wherein the sizes of the rasters scanned on the screen by the external magnetic deflection yoke are different for the middle and outer beams of the three-beam gun, due to the eccentricity of the outer beams in the yoke field, the electron gun is provided with two shield rings 89 of high magnetic permeability, e.g., an alloy of 52 percent nickel and 48 percent iron, known as 52 metal, are attached to the base 81, with each ring concentrically surrounding one of the outer apertures 87, as shown in FIGS. 4 and 7. These magnetic shields 89 by-pass a small portion of the fringe deflection fields in the path of the outer beams, thereby making a slight reduction in the rasters scanned by the outer beams on the screen. The shield rings 89 may have an outer diameter of I50 mils, an inner diameter of 100 mils, and a thickness of 10 mils.

A further correction for this coma distortion is made by mounting two small discs 91 of magnetic material, e.g., that referred to above, on each side of the middle beam path 20a. These discs 91 enhance the magnetic flux on the middle beam transverse to the plane of the three beams and decrease the flux in that plane, in the manner described in the Barkow patent referred to above. The discs 91 may be rings having an outer diameter of mils, an inner diameter of 30 mils, and a thickness of 10 mils.

Each of the electrodes 27, 29, 31 and 33 are mounted on the two glass rods 23 by edge portions embedded in the glass. The two rods 23 extend forwardly beyond the mounting portion of electrode 33, as shown in FIG. 3. In order to shield the exposed ends 93 of the glass rods 23 from the electron beams, the shield cup 35 is formed with inwardly-extending recess portions 95 into which the rod ends 93 extend. The electron gun 19 is mounted in the neck 5 at one end by the leads (not shown) from the various electrodes to the stem terminals 97, and at the other end by conventional metal bulb spacers (not shown) which also connect the final electrode 33 to the usual conducting coating on the inner wall of the funnel 7.

I claim:

1. An in-line electron gun structure for producing three electron beams toward the target in a cathode ray tube, said gun structure comprising;

a. first electrode means for producing and directing three electron beams along spaced co-planar paths having a common general direction; and

b. second electrode means spaced along said beam paths between said first gun structure and said target for forming a separate focusing lens field in the path of each beam, wherein the lens field for the middle beam is symmetrical with the beam path for focusing only, and the two outer lens fields for the outer beams are asymmetric with respect to the other beam paths; said second electrode means comprising a pair of close-spaced electrode plate structures, each having three apertures therethrough, the middle aperture of each plate structure being aligned with the middle beam path, the outer apertures of the plate structure remote from the target being aligned with the respective outer beam paths, the outer apertures of the other plate structure being slightly offset outwardly relative to the respective outer beam paths.

2. In a cathode ray tube including an evacuated envelope containing a screen of repeditive groups of different color emitting phosphor elements at one end and an in-line electron gun at the other end for generating and directing three electron beams along eo-planar paths toward said screen; said electron gun comprising:

a. means for generating three electron beams along three co-planar paths towards said screen; and

b. first and second plate-like focusing grids spaced along said beam paths from said beam generating means in the order named, said first grid having three beam apertures respectively aligned with said three beam paths, said second grid having a middle aperture aligned with the middle one of said three beam paths and two outer apertures slightly offset outwardly with respect to the outer ones of said beam paths, the middle apertures of said grids being adapted when energized to provide a focusing electric field for said middle beam, and the corresponding offset outer apertures of said grids being adapted when energized to provide asymmetric electric fields for individually focusing said two outer beams and for converging them with the middle beam.

3. In a color picture tube including an evacuated envelope comprising a faceplate and a neck connected by a funnel, a mosaic screen of repeditive groups of different color emitting phosphor elements on the inner surface of said faceplate, a multi-apertured color selection electrode spaced from said screen having each aperture aligned with a particular group of phosphor elements and an in-line electron gun mounted in said neck for generating and directing three electron beams along co-planar paths through said electrode to said screen; said electron gun comprising:

a. means for generating three electron beams along three co-planar paths towards said screen; and

b. first and second plate-like focusing grids spaced along said beam paths from said beam generating means in the order named, said first grid having three beam apertures respectively aligned with said three beam paths, said second grid having a middle aperture aligned with the middle one of said three beam paths and two outer apertures slightly offset outwardly with respect to the outer ones of said beam paths, the middle apertures of said grids being adapted when energized to provide a focusing electric field for said middle beam, and the corresponding offset outer apertures of said grids being adapted when energized to provide asymmetric electric fields for individually focusing said two outer beams and for converging them with the middle beam.

4. The structure of claim 3, wherein said beam generating means comprises three cathodes positioned sideby-side in a common plane, each cathode having an electron-emissive surface facing said grids and initiating said co-planar paths, and at least one plate-like grid positioned close to said surfaces and having three apertures, small compared to the apertures of said firstnamed grids, aligned with said beam paths.

5. The structure of claim 3, wherein at least that portion of said second plate-like grid bounding the middle aperture thereof is substantially cylindrically curved in a direction transverse to the common plane of said beam paths and apertures and concave to said first grid, to minimize the ellipticity of the middle beam spot on the screen caused by interaction of adjacent focusing electric fields 6. The structure of claim 3, wherein said tube includes a deflection zone, located in the vicinity of the junction between said neck and said funnel, wherein said beams are subjected to vertical and horizontal magnetic deflection fields during operation of said tube for scanning said beams horizontally and vertically over said screen; and said electron gun further comprises a shielding member of magnetic material surrounding each of said outer beam paths only, in said deflection zone, for partially shielding only the two outer beams from said magnetic deflecting fields, thereby reducing the size of the rasters scanned by the outer beams without reducing the size of the raster scanned by the middle beam.

7. In a color picture tube including an evacuated envelope containing a faceplate and a neck connected by a funnel, a mosaic screen of repeditive groups of different color emitting phosphor elements on the inner surface of said faceplate, a multi-apertured color selection electrode spaced from said screen having each aperture aligned with a particular group of phosphor elements and an in-line electron gun for generating and directing three electron beams along co-planar paths through said electrode to said screen; said electron gun comprising:

a. means for generating three electron beams along three co-planar paths towards said screen; and

b. first and second plate-like grids spaced along said beams paths from said beam generating means, each grid having three beam apertures substantially aligned respectively with said three beam paths, each pair of aligned apertures being adapted when energized to provide a focusing electric field for one of the three beams, at least the middle portion of said second plate-like grid bounding the middle aperture there-of being substantially cylindrically curved in a direction transverse to the common plane of said beam paths and apertures, and con cave to said first grid, to minimize the ellipticity of at least the middle beam spot on the screen caused by interaction of adjacent focusing electric fields 8. The structure of claim 7, wherein the major portion of said second grid is barrel-shaped, concave to said first grid, with the major axis of the barrel shape in said common plane, whereby the curvature of those portions of said second grid bounding the outer apertures in said transverse direction is smaller than that of said middle portion.

9. A cathode ray tube comprising:

a. a target electrode;

b. first electrode means including cathode means for producing and directing a plurality of electron beams toward said target electrode along a plurality of paths which are substantially parallel adjacent to said cathode means; and

0. second electrode means comprising two electrode plates, each having a plurality of beam apertures therein through which said beam paths extend, for forming a plurality of substantially separate electric field lenses, one in each of said paths for focusing said beams near said target electrode; the centers of two of said apertures in the electrode plate closer to said target electrode being spaced further apart than the corresponding centers of two apertures in the other electrode plate for producing asymmetry in at least one of said lens for bending its associated beam toward another of said beams to provide beam convergence.

10. A cathode ray tube comprising:

a. a target electrode;

b. first electrode means including cathode means for producing and directing a plurality of electron beams toward said target electrode along a plural ity of paths which are substantially parallel adjacent said cathode means;

0. second electrode means between said first electrode means and said target electrode for forming a substantially separate electric field lens in the path of each beam for focusing said beams near said target electrode, said second electrode means comprising two adjacent electrode plates having corresponding beam apertures therein through which said beam paths extend, and at least one of said electric field lenses being asymmetric for bending the beam passing therethrough toward at least one other beam; and

. at least one of said plates in said second electrode means being contoured for compensating for astigsaid beams. 

1. An in-line electron gun structure for producing three electron beams toward the target in a cathode ray tube, said gun structure comprising; a. first electrode means for producing and directing three electron beams along spaced co-planar paths having a common general direction; and b. second electrode means spaced along said beam paths between said first gun structure and said target for forming a separate focusing lens field in the path of each beam, wherein the lens field for the middle beam is symmetrical with the beam path for focusing only, and the two outer lens fields for the outer beams are asymmetric with respect to the other beam paths; said second electrode means comprising a pair of close-spaced electrode plate structures, each having three apertures therethrough, the middle aperture of each plate structure being aligned with the middle beam path, the outer apertures of the plate structure remote from the target being aligned with the respective outer beam paths, the outer apertures of the other plate structure being slightly offset outwardly relative to the respective outer beam paths.
 2. In a cathode ray tube including an evacuated envelope containing a screen of repeditive groups of different color emitting phosphor elements at one end and an in-line electron gun at the other end for generating and directing three electron beams along co-planar paths toward said screen; said electron gun comprising: a. means for generating three electron beams along three co-planar paths towards said screen; and b. first and second plate-like focusing grids spaced along said beam paths from said beam generating means in the order named, said first grid having three beam apertures respectively aligned with said three beam paths, said second grid having a middle aperture aligned with the middle one of said three beam paths and two outer apertures slightly offset outwardly with respect to the outer ones of said beam paths, the middle apertures of said grids being adapted when energized to provide a focusing electric field for said middle beam, and the corresponding offset outer apertures of said grids being adapted when energized to provide asymmetric electric fields for individually focusing said two outer beams and for converging them with the middle beam.
 3. In a color picture tube including an evacuated envelope comprising a faceplate and a neck connected by a funnel, a mosaic screen of repeditive groups of different color emitting phosphor elements on the inner surface of said faceplate, a multi-apertured color selection electrode spaced from said screen having each aperture aligned with a particular group of phosphor elements and an in-line electron gun mounted in said neck for generating and directing three electron beams along co-planar paths through said electrode to said screen; said electron gun comprising: a. meAns for generating three electron beams along three co-planar paths towards said screen; and b. first and second plate-like focusing grids spaced along said beam paths from said beam generating means in the order named, said first grid having three beam apertures respectively aligned with said three beam paths, said second grid having a middle aperture aligned with the middle one of said three beam paths and two outer apertures slightly offset outwardly with respect to the outer ones of said beam paths, the middle apertures of said grids being adapted when energized to provide a focusing electric field for said middle beam, and the corresponding offset outer apertures of said grids being adapted when energized to provide asymmetric electric fields for individually focusing said two outer beams and for converging them with the middle beam.
 4. The structure of claim 3, wherein said beam generating means comprises three cathodes positioned side-by-side in a common plane, each cathode having an electron-emissive surface facing said grids and initiating said co-planar paths, and at least one plate-like grid positioned close to said surfaces and having three apertures, small compared to the apertures of said first-named grids, aligned with said beam paths.
 5. The structure of claim 3, wherein at least that portion of said second plate-like grid bounding the middle aperture thereof is substantially cylindrically curved in a direction transverse to the common plane of said beam paths and apertures and concave to said first grid, to minimize the ellipticity of the middle beam spot on the screen caused by interaction of adjacent focusing electric fields.
 6. The structure of claim 3, wherein said tube includes a deflection zone, located in the vicinity of the junction between said neck and said funnel, wherein said beams are subjected to vertical and horizontal magnetic deflection fields during operation of said tube for scanning said beams horizontally and vertically over said screen; and said electron gun further comprises a shielding member of magnetic material surrounding each of said outer beam paths only, in said deflection zone, for partially shielding only the two outer beams from said magnetic deflecting fields, thereby reducing the size of the rasters scanned by the outer beams without reducing the size of the raster scanned by the middle beam.
 7. In a color picture tube including an evacuated envelope containing a faceplate and a neck connected by a funnel, a mosaic screen of repeditive groups of different color emitting phosphor elements on the inner surface of said faceplate, a multi-apertured color selection electrode spaced from said screen having each aperture aligned with a particular group of phosphor elements and an in-line electron gun for generating and directing three electron beams along co-planar paths through said electrode to said screen; said electron gun comprising: a. means for generating three electron beams along three co-planar paths towards said screen; and b. first and second plate-like grids spaced along said beams paths from said beam generating means, each grid having three beam apertures substantially aligned respectively with said three beam paths, each pair of aligned apertures being adapted when energized to provide a focusing electric field for one of the three beams, at least the middle portion of said second plate-like grid bounding the middle aperture there-of being substantially cylindrically curved in a direction transverse to the common plane of said beam paths and apertures, and concave to said first grid, to minimize the ellipticity of at least the middle beam spot on the screen caused by interaction of adjacent focusing electric fields.
 8. The structure of claim 7, wherein the major portion of said second grid is barrel-shaped, concave to said first grid, with the major axis of the barrel shape in said common plane, whereby the curvature of those portions of said second grid bounding the outer apertures in said transverse direction is smaller than that of said middle portion.
 9. A cathode ray tube comprising: a. a target electrode; b. first electrode means including cathode means for producing and directing a plurality of electron beams toward said target electrode along a plurality of paths which are substantially parallel adjacent to said cathode means; and c. second electrode means comprising two electrode plates, each having a plurality of beam apertures therein through which said beam paths extend, for forming a plurality of substantially separate electric field lenses, one in each of said paths for focusing said beams near said target electrode; the centers of two of said apertures in the electrode plate closer to said target electrode being spaced further apart than the corresponding centers of two apertures in the other electrode plate for producing asymmetry in at least one of said lens for bending its associated beam toward another of said beams to provide beam convergence.
 10. A cathode ray tube comprising: a. a target electrode; b. first electrode means including cathode means for producing and directing a plurality of electron beams toward said target electrode along a plurality of paths which are substantially parallel adjacent said cathode means; c. second electrode means between said first electrode means and said target electrode for forming a substantially separate electric field lens in the path of each beam for focusing said beams near said target electrode, said second electrode means comprising two adjacent electrode plates having corresponding beam apertures therein through which said beam paths extend, and at least one of said electric field lenses being asymmetric for bending the beam passing therethrough toward at least one other beam; and d. at least one of said plates in said second electrode means being contoured for compensating for astigmatic distortion caused by the interaction of adjacent electric field lenses to produce substantially circular beam spots.
 11. A cathode ray tube as in claim 10, wherein said gun structure constitutes an in-line gun, said first and second electrode means are adapted to produce, direct, focus and converge three electron beams along three co-planar paths, and said compensating contour comprises a substantially cylindrical curvature of the electrode plate closer to said target, concave to the other electrode plate, in a direction transverse to the plane of said beams. 